Chaotic signal enabled low probability intercept communication

Abstract

A circuit for generating chaotic signals implemented using heterojunction bipolar transistors (HBTs) and utilized in low probability intercept communications. The HBT chaotic circuit generates truly random analog signals in the GHz range that are non-repeating and deterministic and may not be replicated by preloading a predetermined sequence. A fully differential autonomous chaotic circuit outputs two pairs of chaotic signals to be used in a communication system. As it is impossible to generate identical chaotic signals at the transmitter and receiver sites, the receiver itself sends the chaotic signal to be used for encoding to the transmitter. The receiver includes a chaotic signal generator and digitizes, upconverts, and transmits the generated chaotic signal to the transmitter. The transmitter uses the received chaotic signal to code data to be transmitted. The receiver decodes the transmitted data that is encoded by the chaotic signal to retrieve the transmitted data.

1. An autonomous chaotic circuit having a first pair of circuit output terminals and a second pair of circuit output terminals and being fully differential, the autonomous chaotic circuit comprising:
 a first circuit having a pair of first input terminals and a pair of first output terminals, the pair of first input terminals coupled to the first pair of circuit output terminals in a positive feedback configuration; 
 a multiplier circuit coupled to the first circuit in a positive feedback configuration, the multiplier circuit having a pair of multiplier input terminals and a pair of multiplier output terminals, the pair of multiplier input terminals being coupled to the pair of first input terminals, and the pair of multiplier output terminals being coupled to the pair of first output terminals forming a first pair of common output terminals including a first common output terminal and a second common output terminal; 
 a first LC circuit having a first inductor coupled to a first capacitor in parallel, the first LC circuit being coupled to a reference voltage at one terminal; 
 a second LC circuit having a second inductor coupled to a second capacitor in parallel, the second LC circuit being coupled to the reference voltage at one terminal; 
 a first resistor coupled between the first LC circuit and the first common output terminal, a node between the first resistor and the first LC circuit being a third common output terminal; and 
 a second resistor coupled between the second LC circuit and the second common output terminal, a node between the second resistor and the second LC circuit being a fourth common output terminal, the third common output terminal and the fourth common output terminal forming a second pair of common output terminals; 
 wherein the first circuit and the multiplier circuit are implemented using HBT, and 
 wherein the first inductor, the second inductor, the first capacitor, and the second capacitor are implemented by passive inductors and passive capacitors. 
 
   
   
     2. The autonomous chaotic circuit of  claim 1 , further comprising:
 a first pair of emitter followers coupled to the first pair of common output terminals; 
 a second pair of emitter followers coupled to the second pair of common output terminals; 
 a first output buffer coupled to the first pair of emitter followers; and 
 a second output buffer coupled to the second pair of emitter followers, 
 wherein the first pair of emitter followers, the second pair of emitter followers, the first output buffer, and the second output buffer are implemented using HBT. 
 
   
   
     3. The autonomous chaotic circuit of  claim 1 , wherein the first circuit includes:
 a pair of first circuit transistors each having an emitter, a collector, and a base; 
 a pair of first circuit resistors coupled in a differential configuration to the pair of first circuit transistors, two transistors in the pair of first circuit transistors each being coupled between a first node and an emitter of one of the transistors in the pair of first circuit transistors; and 
 a first circuit current source coupled to the first node, 
 wherein each first input terminal in the pair of first input terminals is coupled to a base of one of the two transistors in the pair of first circuit transistors, and 
 wherein each first output terminal in the pair of first output terminals is coupled to a collector of one of the two transistors in the pair of first circuit transistors. 
 
   
   
     4. The autonomous chaotic circuit of  claim 1 ,
 wherein the HBT transistors implementing first circuit and the multiplier circuit have a cutoff frequency of substantially 200 GHz, 
 wherein the first resistor and the second resistor each have a resistance of substantially 130Ω, 
 wherein the first inductor and the second inductor each have an inductance of substantially 2.5 μH, 
 wherein the first capacitor and the second capacitor each have a capacitance of substantially 2.75 pF, 
 wherein the first circuit includes a current source producing a current of substantially 1.6 mA, and 
 wherein the multiplier circuit includes a current source producing a current of substantially 5.1 mA. 
 
   
   
     5. The autonomous chaotic circuit of  claim 1 ,
 wherein the autonomous chaotic circuit has a first differential pair of circuit output voltages at the first pair of circuit output terminals and a second differential pair of circuit output voltages at the second pair of circuit output terminals, 
 wherein the first circuit is receiving the first differential pair of circuit output voltages at the first input terminals and providing a differential pair of first currents at the first output terminals, 
 wherein the multiplier circuit is receiving the first differential pair of circuit output voltages at the multiplier input terminals and providing a differential pair of multiplier currents at the multiplier output terminals, and 
 wherein the differential pair of first currents and the differential pair of multiplier currents are added together at the first pair of common output terminals. 
 
   
   
     6. A method for autonomous and differential generation of a pair of chaotic signals, the method comprising:
 generating a differential pair of currents by supplying a differential control voltage and positive feed back to a negative resistance circuit, the negative resistance circuit implementing a negative resistance and having nonlinear input-output characteristics; 
 generating resonance by providing the differential pair of currents to a pair of parallel LC circuits through a pair of resistors; 
 obtaining a first chaotic signal from a differential pair of voltages output by the negative resistance circuit at nodes before the pair of resistors; and 
 obtaining a second chaotic signal from the differential pair of voltages output by the negative resistance circuit at nodes after the pair of resistors, 
 wherein the negative resistance circuit includes a parasitic capacitance, and 
 wherein the resonance generated is substantially in the GHz range. 
 
   
   
     7. The method of  claim 6 , wherein the pair of parallel LC circuits are implemented using passive inductors and passive capacitors.